Poison Plants and Your Horse

Plant poisonings are rare.  However, when they do occur, the results can be disastrous.  In most instances, horses will avoid poisonous plants and weeds as they will generally taste bad.  In most cases it is reasonable to assume that poisonous plants will exist where your horse lives and it is also reasonable to assume that most of the time, your horse will avoid eating them.  Problems tend to occur when your pasture gets overgrazed.  In this circumstance, poisonous plants and weeds may proliferate and the chance of your horse inadvertently ingesting them increases.  The effect of poisonous plants may be instantaneous while others may cause problems over hours to days after ingestion.

Poisonous Plants

• Poison Hemlock – Found throughout Canada, this weed will grow on roadsides, in ditches. Ingestion of this plant will cause frothing at the mouth, and weakness which will progress to convulsions and death in a very short period of time.  Supportive veterinary care can counter the effects of poisoning and requires emergency care.
• Alsike Clover – Found throughout Canada, this weed will grow in very cold northern climates and in heavy clay soils.  This can be found regularly in pastures and in hay.  Provinces like Alberta can have significant problems with this contaminating hay.  Photosensitivity (an allergy like reaction to sun exposure) is a common effect.  Chronic exposure can cause depression, blindness, staggering, and kidney/liver failure. 
• Buttercup – Found throughout Canada, this weed will grow in pastures and along fence lines and can be found in hay.  When eaten fresh, it will cause gastrointestinal upset but is considered non-toxic when dried in hay.  Symptoms can be mild (salivation) to severe (colic, bloody urine, diarrhea, respiratory problems, weak pulse and twitchy eyelids.  Chronic exposure can be a real problem. 
• Tansy Ragwort -  Found throughout Canada, particularly in pastures, fencelines, and on roadsides.  Consumed in small amounts this may cause weakness, appetite and weight loss, and liver problems.  If consumed continually over 2-3 weeks, liver failure can ensue. 
•  Field Horsetail – Found throughout Canada, particularly in sandy/gravely areas with good drainage.  This weed is especially poisonous to young horses and will increase in toxicity when dried and found in hay.  Liver problems are common and horses will develop jaundice.  Loss of appetite, weakness, excitability, paralysis and staggering gait are common. 
• Japanese Yew Tree – Can grow anywhere and are commonly used as ornamental species.  All parts of the tree are poisonous so these should not be located where the horse may chew bark or consume leaves.  A very small amount 0.5lbs for a 1000lb horse is fatal.
• Red Maple – Found primarily in eastern Canada but will grow in the west.  Horses are at risk primarily from ingesting wilted leaves.  This can cause damage to red blood cells which then goes on to damage the kidneys.  A blood transfusion may be required if severe enough.  Regular Maple trees found in the west do not cause this problem. 
• Black Walnut – Found throughout Canada, the concern is predominantly for exposure to wood.  When shavings originate from hardwood mills, there is a chance that they may contain Black Walnut.  Simply standing on these shavings will result in development of laminitis which may not be reversible.  

Preventing poisoning is always the best strategy.  This can be achieved by carefully managing the use of a pasture so as to not allow overgrazing and to cultivate the pasture to allow for high quality grass growth only.  Good quality orchard grass that is not overgrazed will out compete weed species.  If overgrazing is prevented, temptation to graze on weeds at the fence line will be minimized.  Use of roundup to kill weed species in the pasture and on fence lines should be performed regularly (remember to keep your horses from these fields for a few days after spraying).  If poisonous trees are close to your fields or pastures, you will need to consider removing these trees.

 
  Alsike Clover

 Buttercup

 Field Horsetail

  Poison Hemlock

 Tansy Ragwort